This invention relates generally to implosion protection bands for cathode-ray tubes (CRTs) and particularly to a method of forming a shrinkfit implosion protection band.
A cathode-ray tube is evacuated to a very low internal pressure and accordingly is subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the tube. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply a compressive force to the sidewall of the CRT to redistribute some of the faceplate forces. The redistribution of the faceplate forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the faceplate. An implosion protection band is also beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension and the band causes compression in faceplate areas which otherwise would be in tension. Additionally, in the event of an implosion the redistributed stresses cause the imploding glass to be directed toward the back of the cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured..
An implosion protection band of the shrinkfit type typically is manufactured by forming a strip of steel into a loop having the same configuration as the faceplate to be protected and joining the two ends of the strip on one side of the band. In some instances, the band is made by joining two identical strips on two sides to form the loop. For both types of bands, the periphery of the loop is slightly smaller than the periphery of the faceplate. The loop is heated to approximately 300.degree. to 500.degree. C. and the coefficient of expansion of the material causes the loop to expand to dimensions permitting the loop to be slipped around the sides of the faceplate. As the band cools it shrinks and tightly surrounds the faceplate, thereby applying the necessary implosion protection compression to the faceplate sidewall. The compressive force can be accurately controlled by exceeding the yield point of the metal in the band.
The ends of the strips are permanently joined by either welding or crimping. In either event, because the strip is used to apply substantial pressure to the sidewall of the tube, it is essential that the joint formed when the two ends are coupled together be sufficiently strong to withstand the pressure. It is therefore important to test the integrity of the joint prior to applying the band to a CRT. It is also important to prepare the loop in a manner which assures that the loop will properly seat on to the sidewall of the CRT and will apply optimum compressive forces to the CRT. Additionally, it is necessary that where mounting lugs are attached to the band for securing the tube within a receiver, the lugs cooperate with the band to improve the integrity thereof. The present invention fulfills these important criteria.